The two beady eyes set on either side of a snake’s head will never blink, never close. That’s because they are covered in clear, immovable scales known as spectacles. But unlike the specs that you or I might wear, these are filled with blood.

Evolution has fused snakes’ eyelids to protect their eyes and prevent them from drying out. But their spectacles look clear, they aren’t entirely so. Kevin van Doorn, a snake researcher at the University of Waterloo in Canada, discovered the trait when he was studying how the reptiles focus. He had been shining a bright light on a snake’s eyes to illuminate the process when he noticed a web of blood vessels running through the spectacle scale.

“I never knew that snakes had blood vessels in their spectacle,” van Doorn says. Early literature makes a references these structures as far back as 1852, but this knowledge seems to have been largely forgotten. “In fact, it seems like most vision researchers and even snake researchers, herpetologists, in general had no idea,” he says.

Blood is coursing over the eyes of this coachwhip snake.

Such a vasculature seemed improbable to van Doorn when he first discovered it, as the transparent part of most eyes are generally free of blood vessels—after all, coursing blood could obscure vision. But they weren’t completely clear in his coachwhip snakes. And it appeared to change from one moment to the next. Here’s BBC Nature:

While inspecting the spectacles of coachwhip snakes in the lab, Dr van Doorn found that blood flow was indeed dynamic and not constant.

“Instead, there are cycles of flow and no flow – dilation and constriction of the vessels – in resting animals, such that, for a significant proportion of the time, the absence of blood cells within the vasculature means that the clarity of their vision is likely spared,” he said.

Instances of vasoconstriction—and clear vision—seem to be a response to a perceived threat. Van Doorn first noticed it when he turned toward one his snakes after having his back to it for several minutes. The snake, apparently feeling threatened by van Doorn’s about face, restricted blood flow through its spectacle scales. He doesn’t yet know if the response is a sympathetic, or automatic, response to fear or if it’s something the snakes consciously do.

Snakes apparently can’t keep their specs blood-free for indefinite periods, but they are able to shorten the duration of vessel dilation, keeping their vision clearer for longer—the better to see you with.